This invention is related to a microwave circuit comprising a two-sided substrate having a lead-through structure for interconnecting a first microstrip conductor segment on the upperside of the substrate with a second microstrip conductor segment on the underside of the substrate. The invention further relates to a sealed design of such a microwave circuit, in which the sealing is obtained by means of a metal lid which is arranged on the substrate. Furthermore the invention is related to a use of such a microwave circuit as a function module unit in a circuit arrangement which is built by modules.
Microwave signals are the designation of electrical signals in the frequency range 0.5-50 GHz. Microwave circuits for handling such microwave signals generally consist of a substrate of an insulating material, commonly aluminum oxide. By sputtering followed by plating of a pattern on the substrate transmission conductors, resistors and optionally capacitors may be built. Remaining components are soldered or glued onto the substrate.
A general problem in microwave circuits of the type in question is that the high signal frequencies involved demand electrical matching at existing interfaces in the circuit, meaning that channels in which signals are transmitted and meeting each other at said interfaces shall have identical impedances, customarily 50 Ohm. For example, interfaces of this kind appear between inherent circuit elements and between different conductor sections. Impedance differences at said interfaces give rise to partial reflection of the signal and a consequent loss of power. The loss of power increases with an increasing frequency. The signal power is expensive at the high frequency levels involved, specifically in applications requiring a high output power level. From this it follows that the quality demands on matching are very high.
The provision of good matching at all interfaces in a microwave circuit means very often that difficult problems must be solved by the circuit design. Specifically, this problem is prominent when using a two-sided substrate in which the signal path for external connection of the circuit includes a metal plated hole, a so called through-hole, passing the substrate and interconnecting microwave components on the upperside of the substrate with an external connection terminal on the underside of the substrate. Said through-hole introduces discontinuities in the impedance level of the signal path. The through-hole shows an increased characteristic impedance due to the fact that an adjoining groundplane is missing. This part of the transmission line corresponds to an inductive series connected element. Furthermore, the conductors which are connected to said hole cross over a groundplane free area immediately adjoining the through-hole, said conductors introducing further inductive series elements.
Prior art in this field teaches the introduction of a compensating capacitive parallel element adequately positioned along the transmission line in the neighborhood of said hole by decreasing the distance between conductor and ground, widening of the microstrip conductor over a short distance, or a parallel connection of a distributed or a concentrated capacitive element of adequate design.
This compensation technology means a consequent introduction of a lowpass filter structure having an upper frequency limit restricting the usefulness of the lead-through.
One example of this prior art in a microwave circuit of the kind mentioned in the introduction has been disclosed in U.S. Pat. No. 4,626,805.
The disclosed microwave circuit (24) comprises a two-sided substrate (26), carrying on the upperside the microwave components of the microwave circuit and the transmission line for interconnecting the same and external connection of the microwave circuit, the underside being provided with a ground plane, coplanar conductors and external connection terminals. The external connection is provided by means of a through-hole (40) in said substrate which interconnects the transmission line (36) with said coplanar transmission line (30). The external connection terminals have connection pins (19), normally provided by means of a so called lead-frame, being soldered to the underside of the substrate onto its groundplane. By means of a metal lid (25) the microwave circuit (24) may be sealed hermetically and shielded. The microwave circuit is intended for surface mounting on an underlying circuit board (10) which is provided with a transmission line (16) of a width which is adapted to said connecting pins as well as coplanar ground terminals combined therewith.
The circuit thus described shows the problems which have been indicated above because an inductive discontinuity is introduced by said through-hole due to the fact that said through-hole lacks a capacitive coupling to adjoining ground conductors/ground plane. Between the metallization around said through-hole and the adjoining ground plane on the underside of the circuit the ground plane a free area or gap (G) appears, being traversed by the transmission lines connected to said through-hole on the upperside and the underside of the substrate. The transmission line parts which are situated directly above said gap as well as said through-hole form inductive series elements in the transmission between upperside and underside. These inductive elements are compensated by empirically dimensioning the size of the gap on the underside, providing thereby an additional capacitance. Thereby a correction of the characteristic impedance is obtained within a limited frequency range.
One further embodiment has been disclosed in which a corresponding additional inductance is provided by said respective transmission lines, the same being compensated for by the additional capacitance of said gap as well as a widened segment of the upperside transmission line. The disclosed circuit designs are useful for frequencies up to 20 GHz and provide for acceptable power losses within this frequency range. For frequencies above 20 GHz the losses increase rapidly making thereby the circuit practically useless.
SE-9303142-5, which is owned by the Applicant, discloses one further example of this prior art. The microwave circuit disclosed therein comprises a conductor (10) on the upperside of the substrate, traversing a groundplane-less area around the metallization of the through-hole (7) on the underside of the substrate, introducing thereby an additional inductive impedance in correspondence with the circuit according to U.S. Pat. No. 4,626,805, which has been disclosed above. In this case, the compensation is obtained by means of a so called matching pattern which is connected to the groundplane and which is provided on the upperside primarily at said groundplane-less segment of the conductor adjoining the through-hole. By an adequate dimensioning of the gap between said matching pattern and the conductor a compensating additional capacitive impedance is obtained. This solution has the advantage of introducing said compensation at the mismatch source, that is at said segment of the conductor, which allows for a circuit which is useful for frequencies up to at least 26 GHz.
Another example of this prior art has been disclosed in U.S. Pat. No. 5,057,798, disclosing a transmission line from upperside to underside of a substrate, comprising a through-hole and waveguides connected thereto. Compensation means are provided in order to obtain an additional capacitance compensating the additional inductance of the through-hole. Two embodiments have been disclosed, firstly ground from a plurality of nearby through-holes (via-openings) to form a "coaxial structure", secondly, when microstrip waveguides are used, a specifically adapted design of the microstrip line end. The disclosed transmission line design is used for saving substrate area by arranging desirable parts of the transmission line on the underside of the substrate. The transmission line has the disadvantages which have been indicated above because the same include specifically designed compensating means.
Microwave circuits of the actual type as well have to fulfill the requirement of being useful within a wide frequency range, for example from 2 GHz up to 26 GHz, maintaining substantially identical performance in respect of power losses and stable signal level within the complete frequency range. The use of compensating elements according to prior art, contradicts the requirement on a wide frequency range because the undesirable, parasitic additional impedances will vary with the signal frequency in the same way as the additional impedances which are introduced by means of compensating measures, from which follows that the compensation will be optimized within limited frequency intervals.